GB2044477A

GB2044477A - Optical fibres

Info

Publication number: GB2044477A
Application number: GB7943968A
Authority: GB
Original assignee: Heraeus Quarzschmelze GmbH; Heraeus Schott Quarzschmelze GmbH
Current assignee: Heraeus Quarzschmelze GmbH; Heraeus Schott Quarzschmelze GmbH
Priority date: 1979-02-27
Filing date: 1979-12-20
Publication date: 1980-10-15
Also published as: JPS55117104A; NL8000905A; DE2907650A1; DE2907650C3; FR2450466B1; DE2907650B2; GB2044477B; US4392715A; FR2450466A1

Description

1 GB2044477A 1

SPECIFICATION

1 Improvements in or relating to optical fibres The present invention relates to optical fibres having a core, the material of which consists basically of quartz glass, and with a sleeve of plastics material optically effective at least over a length of 10 metres, reckoned from the light-engaging end of the fibre, which has a refractive index n, which is smaller than that of the core.

Optical fibres of this kind are known from and described in United States Patent Application No. 3869194. As opposed to optical fibres whose core and sleeve consist of virtreous material, optical fibres with a plastics sleeve posses the advantage that they are cheaper to produce. The plastics-covered optical fibres with a glass core have a highnumbered exciting aperture in comparison to optical fibres with core and sleeve of vitreous material.

A total reflection of the light supplied to the core of the fibre occurs at the interface between the vitreous material core and the plastics sleeve. The combination of core and sleeve material requires a maximum amount of care; however scatter losses at the interface between core and sleeve material due to faults, dust particles or the like are unavoidable.

Therefore it has also been suggested, for reduction of the scatter losses, to replace the optical fibres with glass core and plastics sleeve by such optical fibres as consist of only a core material. For this purpose, the outer layer of the core material has been so altered by a diffusion process as described in German specification no. 190 1053, that it acts like a sleeve. Then however the optical fibre form, already mentioned in the introduction, occurs which consists only of vitreous materials and has a low-numbered exiting aperture.

It is an object of the invention to provided an optical fibre which, by retaining the highnumbered exciting aperture, ensures better light conductive properties than known plas- tics-covered optical fibres, whereby the advan- 115 tage of cost- favourable reproduction is further maintained.

Accordingly, the invention consists in an optical fibre having a core, the material of which consists basically of quartz glass, and with sleeve of plastics material optically effective at least over a length of 10 meters, reckoned from the light engaging end of the fibre, which flas a refractive index n, which is smaller than that of the core, wherein the core 125 has an outer layer, the refractive index nAus of which is smaller than the constant refractive index n, of the remaining inner part of the core and larger than the refractive index n, of the sleeve, and wherein nAus lies in the area of 130 nK-(nK-nm). (0.25 to 0.8) and the thickness of which lies between 0.8 gm and 8 gm.

An outer layer, the thickness of which is 2 to 6 gm and the refractive index n,u of which is in the area of nK_(nK-nm). (0.4 to 0.6), has proved successful. Optical fibres according to the invention, in which the thickness of the outer layer is 4 gm and refractive index nAus = nK(%-nJ 0.5 give especially good results. The refractive index of the outer layer of the core is advantageously constant over the whole layer thickness. The invention however, also comprises such fibres in which the refractive index of the outer layer reduces with increasing distance from the core axis. The only prerequisite is that the refractive index lies within the area specified. Optical fibres, in which the outer layer of the core consists of fluorine or boron-doped quartz glass, have proved successful. The inner core part of an optical fibre according to the invenion consists of quartz glass or quartz glass doped with means increasing the refractive index. As such means for increasing the refractive index, ger- manium, phosphor, titanium, and aluminium compounds are used, as is known per se. If the inner core part of the fibre consists of quartz glass doped with means for increasing the refractive index, an undoped quartz glass layer may be used as the outer layer.

Optical fibres formed according to the invention possess the advantage that, at the outer layer of the core, the refractive count and thickness of which satisfies the claimed conditions, a large part of the engaged light is totally reflected at the interface between the outer layer of the core and the plastics sleeve. The last mentioned proportion is indeed no longer as high as in the known optical fibres with quartz glass core and plastics sleeve; it is, however, not negligibly low, which means that the plastics sleeve of an optical fibre according to the invention has an optical effect. Fibres according to the invention have an approximately equally high-numbered exciting aperture like the known plastics covered quartz glass fibres, which corresponds to the refractive index difference between the refractive index of the inner part of the core and the plastics sleeve. Optical fibres with the special outer core layer have a higher light output than known optical fibres with quartz glass core and plastics sleeve, which, calculated over a length of 1 km, is approximately 50% higher than that of the known plastics covered quartz glass fibre.

Silicon resins such as, for example, the silicon resin of the firm ShinEtsu Chemicals fE 103 RTV and polytetrafluoroethylene, have proved especially successful as material for the plastics sleeve.

Known methods of manufacture for plasticscovered quartz glass fibres can be used for the production of optical fibres according to the invention as described, for example, in GB2044477A 2 Figs. 8 and 9 of United States Patent Specification No.3869194 mentioned above. As raw material a rod, the inner core part of which consists of quartz glass and the outer layer of which consists of fluorine-doped glass, can be used in place of a quartz glass rod. Such raw materials can, for example, be produced in the way described in German Specification no. 2536456.

In order that the invention may be more clearly understood reference will now be made to the accompanying drawings which show one embodiment thereof by way of example and a method for its manufacture, and in which:

Fig. 1 is a longitudinal section through an optical fibre according to the invention, and Fig. 2 schematically shows a method of manufacturing the fibre shown in Fig. 1.

Referring now to the drawings, in Fig. 1 the inner core part referred to above is designed by reference numeral 1. It consists of quartz glass and, specifically of synthetic quartz glass obtained from gaseous silicium halogenides, which has less than 10 ppm OP-ions and in the immediate infra-red spectral area has total optical losses of less than 4 dB/km, measured as a whole. The outer layer 2 of the core consists of fluorine-doped synthetic quartz glass with a fluorine-ion content of 30.000 ppm fluorine ions and less than 10 ppm OHions. A plastics sleeve which consists of silicon resin is shown at 3.

The manufacture of such a fibre is ex- plained with reference to the schematic Fig. 2. A rod-shaped semi- finished rod is shown at 4 which consists of an inner core part of synthetic quartz glass and an outer part of fluorine-doped synthetic quartz glass (the re- maining material properties of inner core part and outer layer correspond to the details in Fig. 1). The rod 4 is passed within a heating device 5 and is heated there to drawing temperature. The drawn fibre then passes through a plastics-coating device 6 in which the intended sleeving material of silicon resin is contained in a liquid condition. The fibre emerging from the device 6 then has a structure as shown in Fig. 1 - It can then be wound onto a drum 7 as is usual in the art. If an extruder is used for the plastics-coating device, no age-hardening device is connected between the coating device and the winding drum 7 as is represented in the embodiment by device 8.

Claims

1. An optical fibre having a core, the material of which consists basically of quartz glass, and with a sleeve of plastics material optically effective at least over a length of 10 metres, reckoned from the light engaging end of the fibre, which has a refractive index % which is smaller than that of the core, wherein the core has an outer layer, the refractive index n,,, of which is smaller than the constant refractive index n, of the remaining inner part of the core and larger than the refractive index n, of the sleeve, and wherein nAus lies in the area of n,-(n,-nm). (0.25 to 0.8) and the thickness of which lies between 0.8 gm and 8 gm.

2. A fibre as claimed in claim 1, wherein the refractive index of the outer layer of the core is constant over the entire layer thickness.

3. A fibre as claimed in claim 1, wherein the refractive index of the outer layer of the core decreases with increasing distance from the axis of the core.

4. A fibre as claimed in claim 1, 2 or 3, wherein the outer layer of the core consists of fluorine or boron quartz glass.

5. A fibre as claimed in any one of claims 1 to 4, wherein the inner part of the core consists of quartz glass dosed with means for increasing the refractive index.

6. A fibre as claimed in claim 1 or 2, wherein the outer layer of the core consists of quartz glass and the inner part of the core consists of quartz glass doped with means for increasing the refractive index.

7. A fibre as claimed in any of the preceding claims, wherein the quartz glass of the inner part of the core and of the outer layer is a synthetic quartz glass produced from silicon halide, the OH content of which is less than 10 ppm.

8. A fibre as claimed in any of the preced- ing claims, wherein the outer layer has a refractive index n,,,s which is in the area of nK-(n,-n,). (0.4 to 0.6) and the thickness of which is 2 to 6 gm.

9. A fibre as claimed in any of the preced- ing claims, wherein the plastics sleeve consists of silicon resin or polytetrafluoroethylene.

10. An optical fibre substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

11. A method of manufacturing an optical fibre, substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.

Printed for Her Majesty's Stationery Office by Burgess F Son (Abingdon) Ltd.-1 980. Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.

-F4'.